Approaches to controlling disease-carrying insect populations might include inhibiting the development of virus in the mosquito or altering the mosquitoes' lifespan so that they die before they can transmit disease. A major challenge is ensuring that such strategies are effective, safe, and socially and environmentally acceptable.

Scientists have long known that only relatively old mosquitoes can transmit the agents that cause certain diseases, including dengue fever and malaria. Dr. O’Neill and his multinational team are working on a plan to shorten the lifespan of mosquitoes that transmit the dengue virus, which infects up to 100 million people each year. They are introducing into populations of Aedes mosquitoes, strains of a naturally occurring bacterial symbiont, Wolbachia, that kill infected insects before they are old enough to transmit disease. Wolbachia are inherited though the eggs of the mosquitoes and so are passed on from generation to generation.

Ultimately, the project aims to determine if Wolbachia can be introduced into mosquitoes that transmit the dengue virus, and if so, whether they can reduce mosquito lifespan enough to prevent transmission. To further test their Aedes aegypti lines in realistic conditions, the project team plans to accelerate its work plan and move to field cage experiments as soon as possible. This will allow them to evaluate the likely efficacy of the intervention for this species of mosquito.

Research Objectives:

Transinfect Aedes with Wolbachia strains that have the desired life-shortening effect. Strains to be tested include wMelPop, a rapidly multiplying strain that shortens Drosophila lifespan. Investigators will try to identify the gene or genes responsible for this phenotype in order to optimize the effect by genetic manipulation.

Test the efficiency of the Wolbachia life-shortening strategy under laboratory conditions, including vector competence for dengue transmission

Evaluate natural mosquito populations and disease transmission dynamics in several field sites and conduct a field cage test of efficacy at one site (Queensland)

Establish partnerships with communities in disease-endemic countries (Thailand, Vietnam) in preparation for possible field release trials

Investigators have generated two independent lines of Aedes aegypti, the primary dengue vector, each carrying the wMelPop infection with 100 percent maternal transmission. Investigators are completing laboratory characterization of the two lines, both of which induce strong life-shortening effects under laboratory conditions.

The project team has determined that the two lines induce strong cytoplasmic incompatibility, which is required if the infection is to drive into field populations. Investigators have found infection with Wolbachia also generates a slight fecundity cost for infected female mosquitoes, although this seems to decrease with time in lab culture. Experiments with generations 6 and 12 mosquitoes found that infection halved adult lifespan, indicating that the fecundity cost attenuation seems independent of the life-shortening effect.

Investigators have completed 95 percent of sequencing of the genome of wMelPop in order to identify life-shortening genes in Wolbachia. A transformation system for Wolbachia is under development to test function of putative life-shortening genes. Progress has been made with both phage and homologous recombination approaches.

Investigators are examining whether Wolbachia infections have direct effects on vector competence for dengue viruses. Initial work focuses on dengue2 and Australian strains of Aedes aegypti. The team developed and published a new method to age-grade Aedes aegypti, which has been used to estimate baseline population age structure of Aedes aegypti in Australia, Thailand, and Vietnam and to provide test efficacy of the intervention in the future.

The team has begun field studies in Australia, Thailand, and Vietnam to characterize local sites and mosquito populations where future interventions may take place.

Investigators' work with the secondary dengue vector, Aedes albopictus, has been less successful. The transferred infection does not appear to be transmitted as well as in Aedes aegypti. Experiments attempting to correct this issue are underway.